A common mode choke (CMC) removes common mode noise from a differential wire pair, and the extent of filtering out the common mode noise is referred to as common mode insertion loss. In data transmission applications that use unshielded twisted pair (UTP) cable, the common mode insertion loss of the CMC used by the data physical link layer devices (PHYs) at each end of the cable is critical to preserving the integrity of the data communications link in the presence of common mode noise.
Common mode insertion loss is limited by several constraints on the CMC such as footprint, inter-winding capacitance, leakage inductance, the DCR of the windings, and core loss. In addition the CMC must have low insertion loss and high return loss as required by the differential data transmitted between the PHYs. For applications using UTP cables that need to achieve very low bit error rates in noisy environments, it may not be feasible for a conventional CMC to achieve the required common mode insertion loss. The solution to this impasse usually entails replacing the UTP cable and connectors with relatively expensive shielded cable and connectors.
Better techniques are needed to remove common mode noise from a wire pair carrying differential data.